1. Field of the Invention
The present invention relates to a method of driving a step motor, and more particularly, to a method of driving a step motor used in office equipment such as printers, photocopying machines, scanners, and facsimiles.
2. Description of the Related Art
A step motor rotates by a predetermined angle, namely a step angle, on application of one pulse. The step motor, with characteristics of high performance, small size, and light weight has been widely used as a core driving source, and demand has been increasing due to developments in mechatronics fields like factory automation (FA), office automation (OA), and precision industry. In particular, the step motor is widely used as the driving source for ink jet printers, scanners, facsimiles, and laser printers that require position control with high precision and low noise level in various speed ranges.
FIGS. 1 and 2 schematically illustrate an example of a conventional driving device using a step motor. Reference numerals 10, 12, 14 and 16 denote first through fourth reduction gears that transfer driving force from a step motor M to a feed roller D1. Reference character D2 denotes a compression roller that compresses a sheet 18 against the feed roller D1 to transfer the sheet 18 into the driving device.
In this case, the step motor M rotates by a predetermined step angle, so that the first reduction gear 10, which is connected to the shaft of the step motor M, is rotated by the same angle. The second reduction gear 12 engaged with the first reduction gear 10 is rotated by an angle corresponding to the amount of linear movement of the first reduction gear 10. The rotation of the second reduction gear 12 transferred to the third and fourth reduction gears 14 and 16 rotates the feed roller D1, so that the sheet 18 moves a distance corresponding to an angle of rotation of the feed roller D1. As a result, the sheet 18 moves the distance calculated by multiplying a gear ratio of the reduction gears 10, 12, 14, and 16 by a predetermined step angle of the step motor M. In this case, a number of teeth of the first reduction gear 10 is divided by a number of teeth of the second reduction gear 12, and a number of teeth of the third reduction gear 14 is divided by a number of teeth of the fourth reduction gear 16. Then, the respective results of the division are multiplied to obtain the gear ratio of the reduction gears 10, 12, 14, and 16. The gear ratio of the first through fourth reduction gears 10, 12, 14, and 16 is fixed, so that the distance of movement of the sheet 18 is determined by controlling the drive of the step motor M.
The step motor M is driven by a step motor driver (not shown) that supplies a predetermined driving current to each phase of the step motor M. In particular, a predetermined driving pulse driving the step motor M is applied from a step motor controller (not shown) to the step motor driver, so that the step motor driver applies the driving current to the step motor M. The step motor M receives the driving current for rotation through a predetermined angle. For example, when there are two current control bits for the step motor M, the step motor M rotates by one-fourth of a step angle, which is changed from one step angle, by controlling the current applied to a phase. In this case, since the step angle of the step motor M is reduced from one to one-fourth, the rotation speed of the step motor is decreased to one-fourth. Moreover, since the one-fourth step angle is formed by dividing the step angle into four, the movement of the sheet 18 at the one-fourth step angle is controlled to be four times more precise as compared to at the one step angle. However, when the step motor M is driven by the one-fourth step angle, the step motor M requires four times the step pulses as compared to at the one step angle to maintain the speed of the sheet 18. The amount of frequencies output from a switching pulse is increased by four times, and the operations related to ports that are performed by the controller like a microcomputer or a central processing unit (CPU), which controls the driving device, are increased by four times.
Accordingly, to obtain an output of high resolution using a conventional driving device with a step motor driven by one step angle, the reduction gear ratio has to be increased, so that a number of reduction gears is increased, thereby increasing a size of the driving device.
When the step motor is driven by 1/n step angle to improve driving speed and precision in position control, vibration noise is generated from the reduction gears when driven at high speed, and the switching pulse may exceed a clock limit of the CPU, since the switching pulse is increased by n times.